Field of the Invention
The present invention relates to an ESD protective device and more particularly, relates to an ESD protective device in which discharge is generated between facing portions of discharge electrodes, the facing portions being exposed in a cavity portion formed inside a ceramic multilayer substrate.
Description of the Related Art
ESD (Electro-Static Discharge) indicates a phenomenon in which intense discharge is generated when a charged conductive object (such as a human body) is brought into contact with or is placed sufficiently close to another conductive object (such as an electronic apparatus). When ESD is generated, problems, such as damage to and malfunction of electronic apparatuses, may arise. In order to prevent the generation of ESD, it is necessary to prevent an excessive voltage generated in discharge from being applied to a circuit of an electronic apparatus. An ESD protective device is a device used for the application as described above and is also called a surge absorbing element or a surge absorber.
The ESD protective device is disposed, for example, between a signal line of a circuit and the ground (earth). Since having the structure in which a pair of discharge electrodes is disposed with a space provided therebetween, the ESD protective device has a high resistance in a normal usage state, and hence, a signal is not allowed to flow to a ground side. On the other hand, when an excessive voltage is applied, such as the case in which a static charge is applied from an antenna of a mobile phone or the like, discharge is generated between the discharge electrodes of the ESD protective device, and as a result, the static charge can be guided to the ground side. By this flow of the static charge, the voltage caused by the static charge is not applied to circuits provided at the downstream side of the ESD protective device, and hence the circuits can be protected.
For example, in an ESD protective device 1 shown in a cross-sectional view of FIG. 12, a cavity portion 3 and discharge electrodes 6 and 8 facing each other with a space 5 provided therebetween are formed inside a ceramic multilayer substrate 2. The discharge electrodes 6 and 8 include facing portions 7 and 9, respectively, which are formed along an inner surface of the cavity 3. The discharge electrodes 6 and 8 extend from the cavity portion 3 to peripheral surfaces of the ceramic multilayer substrate 2 and are connected to external electrodes 6x and 8x, respectively, which are formed outside the ceramic multilayer substrate 2, that is, which are formed on surfaces of the ceramic multilayer substrate 2. The external electrodes 6x and 8x are used to mount the ESD protective device 1. An auxiliary electrode 4 in which an electrically conductive material is dispersed is formed adjacent to the facing portions 7 and 9 of the discharge electrodes 6 and 8 and to a portion at which the space 5 between the facing portions 7 and 9 is formed.
When a voltage having a predetermined value or more is applied to the external electrodes 6x and 8x, discharge is generated between the facing portions 7 and 9 of the discharge electrodes 6 and 8, and an excessive voltage is guided to the ground by this discharge, so that a circuit provided at the downstream side can be protected. When the space 5 between the facing portions 7 and 9 of the discharge electrodes 6 and 8 and the amount and the type of electrically conductive material contained in the auxiliary electrode 4 are adjusted, a discharge starting voltage may be determined (for example, see Patent Document 1).    Patent Document 1: International Publication No. 2008/146514